The present invention relates to a track changing technique and a data recording technique for a rotary type memory device, and in particular to a technique useful for increasing the access speed to a plurality of non-consecutive tracks, within a cylinder.
For example, in an electronic computer system for general use, using a large scale magnetic disk device as an external memory device is known.
In such a magnetic disk device a plurality of coaxial circular tracks are normally disposed in each of a plurality of magnetic disks. These disks are normally rotatably mounted around an axis. Data is normally managed by using each of the cylinders as unit. Such a device is a logical concept constituted by a plurality of tracks equidistant from the center of rotation of the magnetic disks. Relatively high speed access to those tracks is possible only by an operation of selecting a head without any seek operation based on the movement of the head in the radial direction of the magnetic disks.
The so-called CKD (Count, Key, Data) format is the leading recording format for each of individual track.
That is, each of a plurality of records stored in the tracks is composed of a count area, a key area and a data area, which are arranged with a predetermined gap between each other. The count area includes recorded positioning information on the relevant record within the memory device as well as management information such as the lengths of the key area and the data area, etc. The key area includes search information for searching the data area. The data area includes real data.
Preparing a predetermined processing for each of the successive areas is effected when the head passes through each of the gaps.
However, recently, as the disk device is more and more fixed and the capacity thereof increases, the demand for high speed data transfer becomes stronger. To respond to this demand, there is a tendency to remove wait time for rotation due to the processing of the tracks one after another by dealing with a plurality of tracks in a lump (continuous processing) and on the other hand to shorten the processing time as well as to reduce the number of tapes by decreasing the amount of data by the fact that all the tracks are not wholly dumped/restored. However, only the updated track is selectively done in a dumping operation, by which data are shunted from a magnetic disk to a magnetic tape, etc. or a restoring operation, by which the data are returned from the magnetic tape to the original magnetic disk or from another magnetic disk to the disk.
For this purpose dealing with discontinuous tracks on a same cylinder in a lump becomes necessary.
For this reason, in a magnetic disk device, for which the CKD method is adopted as described above, (e.g., a SEEK HEAD command for changing tracks in a same cylinder) a command with multitrack instruction, etc. are used.
However, by the former (i.e., SEEK HEAD command) since receiving a track number for each of the tracks from the central processing unit of higher rank of the subsystem for every execution is necessary, completing this command during a time period within the time when the head on a gap peculiar to the CKD method is not possible. Consequently repositioning the read/write head by the succeeding reading command is necessary and therefore dealing with the succeeding record without wait time for rotation is not possible. In addition, executing this SEEK HEAD command before the reading command is necessary and therefore the overhead thereof cannot be neglected.
On the other hand, the latter (i.e., command with multitrack instruction) executes the head changing operation at the track starting point (index marker), while securing the positioning of the read/write head, and necessarily selects the following adjacent track.
For this reason, to continue the processing for a non-adjacent track, issuing commands with multitrack instruction several times and waiting for rotation until the index for changing as many times is necessary. Unfortunately, as a consequence more waits for rotation take place with increasing distance between the present track and the succeeding target track.
Therefore, according to the prior art technique, the former (i.e. SEEK HEAD command) is used. However since the processing time for executing this command is great, as described above, a large gap behind the track is necessary for absorbing this overhead when changing the track. Further, repositioning the head is also required.
When a data format is adopted, by which the tracks are almost fully utilized to secure the memory capacity of the magnetic disk device, as is the current common practice, useless time for one rotation inevitably takes place for continuously dealing with non-adjacent tracks. This increases the time necessary for transferring and receiving data between the central processing unit and the magnetic disk device.
Further, as a data recording method for avoiding the wait time for rotation in the magnetic disk device, known techniques are disclosed, e.g. in JP-A-63-83967.
That is, according to this technique, the wait time for rotation when moving and changing the head from a track on a certain cylinder to another track on the next cylinder is avoided by setting the position of the index marker of the tracks and shifting it for different cylinders.
However, according to this technique, since the positions of the index markers of the tracks within each of the cylinders are the same, no effect can be obtained for avoiding the wait time for rotation at the head changing within a same cylinder.